The invention relates to a method for producing oxygen by low-temperature separation of air in a distillation column system which comprises a high-pressure column and a low-pressure column; a main condenser which is configured as a condenser evaporator, wherein the liquefaction space of the main condenser is in fluid communication with the top of the high-pressure column and the evaporation space of the main condenser is in fluid communication with the low-pressure column; an argon discharge column which is in fluid communication with an intermediate point on the low-pressure column; an argon discharge column tops condenser which is configured as a condenser-evaporator, wherein the liquefaction space of the argon discharge column tops condenser is in fluid communication with the top of the argon discharge column; an auxiliary column whose bottom region is configured for introduction of gas from the evaporation space of the argon discharge column tops condenser; wherein liquid crude oxygen from the bottom of the high-pressure column is introduced into the auxiliary column; a liquid stream from the high-pressure column or the main condenser is introduced as reflux onto the too of the auxiliary column via a reflux liquid conduit, wherein the liquid stream has a nitrogen content at least equal to that of air.
The principles of low-temperature separation of air generally and the construction of two-column plants specifically are described in the monograph “Tieftemperaturtechnik” [low-temperature technology] by Hausen/Linde (2nd Edition, 1985) and in an article by Latimer in Chemical Engineering Progress (Vol. 63, No. 2, 1967, page 35). The heat-exchanging relationship between the high-pressure column and the low-pressure column of a double column is generally realized by way of a main condenser, in which tops gas from the high-pressure column is liquefied against evaporating bottoms liquid from the low-pressure column.
The distillation column system of the invention may in principle be configured as a classical two-column system having a high-pressure column and a low-pressure column. In addition to the two separating columns for nitrogen-oxygen separation it may comprise further apparatuses for obtaining other air components, in particular noble gases, for example a krypton-xenon obtaining operation.
An “argon discharge column” refers here to a separating column for argon-oxygen separation which does not serve to obtain a pure argon product, but serves to discharge argon from the air to be fractionated in the high-pressure column and the low-pressure column. Its interconnection differs only slightly from that of a classical crude argon column but it contains far fewer theoretical plates, namely fewer than 40, in particular between 15 and 30. Similarly to a crude argon column, the bottom region of an argon discharge column is connected to an intermediate point on the low-pressure column and the argon discharge column is cooled by a tops condenser on the evaporation side of which decompressed bottoms liquid from the high-pressure column is introduced; an argon discharge column does not comprise a bottoms evaporator.
In the invention the main condenser and the argon discharge column tops condenser are configured as condenser-evaporators. The expression “condenser-evaporator” refers to a heat exchanger in which a first, condensing fluid stream enters into indirect heat exchange with a second, evaporating fluid stream. Each condenser-evaporator has a liquefaction space and an evaporation space, which consist of liquefaction passages and evaporation passages respectively. The condensation (liquefaction) of the first fluid stream takes place in the liquefaction space, the evaporation of the second fluid stream in the evaporation space. The evaporation space and the liquefaction space are formed by groups of passages which are in a heat-exchanging interrelationship.
The main condenser may be configured as a single- or multi-level bath evaporator, in particular as a cascade evaporator (as is described in EP 1287302 B1=U.S. Pat. No. 6,748,763 B2 for example) or else as a falling film evaporator. Said condenser may be formed by a single heat-exchanger block or else by a plurality of heat-exchanger blocks arranged in a common pressure vessel.
The distillation column system of an air separation plant is arranged in one or more cold boxes. A “cold box” is herein to be understood as meaning an insulating encasement which completely encompasses a thermally insulated interior with outer walls; plant parts to be insulated, for example one or more separation columns and/or heat exchangers, are arranged in the interior. The insulating effect may be brought about through appropriate configuration of the outer walls and/or by filling the interspace between plant parts and outer walls with an insulating material. The latter version preferably employs a pulverulent material such as perlite for example. Not only the distillation column system for nitrogen-oxygen separation in a low-temperature air separation plant but also the main heat exchanger and further cold plant parts have to be enclosed by one or more cold boxes. The external dimensions of the cold box typically determine the in-transit dimensions for prefabricated plants.
A “main heat exchanger” serves to cool feed air in indirect heat exchange with return streams from the distillation column system. Said heat exchanger can be formed from a single heat exchanger section or a plurality of parallel and/or serially connected heat exchanger sections, for example from one or more plate heat exchanger blocks. Separate heat exchangers used specifically for evaporation or pseudo-evaporation of a single liquid or supercritical fluid without heating and/or evaporation of a further fluid do not belong to the main heat exchanger.
The relative spatial terms “top”, “bottom”, “over”, “under”, “above”, “below”, “next to”, “side by side”, “vertical”, “horizontal”, etc. relate to the spatial alignment of the separation columns in normal operation. An arrangement of two columns or apparatus parts “one above the other” is understood here to mean that the upper end of the lower of the two apparatus parts is situated at lower or identical geodetic height as the lower end of the upper of the two apparatus parts and the projections of the two apparatus parts in a horizontal plane overlap. In particular, the two apparatus parts are arranged exactly one above the other, i.e. the axes of the two columns proceed on the same vertical straight line.
A method of the type specified at the outset and a corresponding plant are known from IPCOM000176762D. Depicted therein in FIG. 3 is an air separation plant comprising a double column composed of a high-pressure column and low pressure columns comprising an argon column and an auxiliary column arranged thereover. The auxiliary column serves to disburden the low-pressure column and is accordingly operated at the same pressure as the corresponding section of the low-pressure column. Gas from the low-pressure column is introduced at the bottom of the auxiliary column.
The invention has for its object to make the method of the type specified at the outset and a corresponding plant more energy-efficient. It relates in particular to air separation plants of particularly large capacity, in particular for obtaining oxygen. Such plants in particular are configured for an air rate of more than 370 000 Nm3/h, preferably more than 1 000 000 Nm3/h.